1. Field of the Invention
The present invention relates to a projection optical system, and more particularly to a projection optical system for short-distance front projection whereby, for example, a large image is projected from a position close to a screen.
2. Description of Related Art
In recent years, projectors employing a digital micromirror device or LCD (liquid crystal display) have been becoming increasingly popular, and various types of them have been proposed. For example, as projection optical systems adopting oblique projection to offer a short projection distance, Patent Document 1 listed below proposes a type in which the last surface is a convex-surfaced mirror; on the other hand, Patent Document 2 to 4 listed below propose a type in which the last surface is a concave-surfaced mirror.
Patent Document 1: JP-A-2001-264634
Patent Document 2: U.S. Pat. No. 6,883,922
Patent Document 3: U.S. Pat. No. 7,086,742
Patent Document 4: U.S. Pat. No. 7,048,388
With the type in which the last surface is a convex-surfaced mirror as proposed in Patent Document 1, short-distance projection makes the angle of incidence of rays on the screen so large that, in front projection and similar applications, as the distance to the screen varies, the position of the projected image greatly varies in the up/down direction. Thus, in cases where, for example, the projection optical system is set up on a desk, as the projection distance increases, the image moves upward over so long a distance that it is difficult to view the image.
On the other hand, with the type in which the last surface is a concave-surfaced mirror as proposed in Patent Documents 2 to 4, it is possible to make the angle of incidence of rays on the screen comparatively small, and it is thus possible to make comparatively small the amount by which the projected image moves as the projection distance varies. Inconveniently, however, including two or more curved-surfaced mirrors, the projection optical systems proposed there have a large dimension in the up/down direction, and thus, when used as a projection optical system for front projection in a state set up on a desk, they obstruct the viewing of the image. Moreover, these projection optical systems are designed for rear projection, and accordingly do not have a focusing mechanism. Thus, in front projection applications, they cannot focus.